A Workplace Exposure Limit is not a target to work towards "safe" only occurs in the absence of the hazard.

The limits are there for you to determine if the material being used is "in control" i.e. you are not exceeding the published value or "out of control" i.e. you need additional measures over and above what you were doing at time of determination to reduce exposure.

If you don't measure you don't know.

Your target should always be zero level of exposure ISFAIRP.

Substances can also have a STEL (15 minute exposure) that needs considering within the days activity.

Hi Andy

In addition to what Roundtuit has said you should never assume that if your within any occupational exposure limit that conditions are safe.

OELs are almost invariably set as a reaction to past negative impacts of exposure, and are established following consultation between various stakeholders, so are usually compromises - usually the employers look for higher limits and, usually, the trade unions want lower ones.

But they also reflect limits that are relatively safe for the majority of the workforce - you might have say people with pre-existing respiratory issues, for whom exposure to some airborne contaminant at the OEL may be far from safe.

Over the passage of occupational health and safety regulatory history in the UK, OELs are in the main a relatively new thing (years or decades rather than a century or more). 

Initially the HSE adopted most OELs from those set in America by the ACGIH - latterly UK OELs have been more influenced by Europe. In each case, the UK has generally lagged behind international thinking on occupational health risks.

....and it is VERY rare for an OEL in the UK to be amended to be less stringent. Usually as greater knowledge becomes available they either stay where they are or get reduced to require more stringent control.

When I started work as a health and safety professional the limit for exposure to asbestos was 2 fibres per millilitre. That limit has been progressively made more stringent over the years. We have far more detailed and extensive data on the effects of exposure to asbestos than the vast majority of workplace occupational health risks.

COSHH requires exposure to be reduced to the lowest level that is reasonably practicable whether or not there is an OEL.

We do have a reasonably sound idea that roughly 13,000 people in the UK die prematurely each year as a result of exposure to occupational health risks and that about 12,000 of those are from respiratory diseases. We don't have very good intelligence as to exactly what contaminants (or mixes of contaminants) many of those people have been exposed to.

No, it is not how much the supplier thinks will be released over an 8 hour period.

The eight hours is a consideration of a typical working day (in reality it should be 7 + a bit when you take out breaks).

The value as Peter indicated is published by the regulator. 

Measurement of the substance is normally conducted over a shorter period.

Bricks & Concrete contain RCS which has a TWA of 0.1mg/m3

In normal use RCS does not get released from bricks or concrete

When someone dry works bricks or concrete then RCS can be released - the quantity needs to be measured, not as what comes away from a single hole or cut but as a total exposure of the employee over time.

This time defined measurement could calculate as less than TWA indicating suitable control or it could be far in excess meaning more controls are needed.

Those controls (as with HAV's or noise) could include limiting the amount of time someone is exposed to the hazard.

The exposure limit is in some respects like a speed limit.

The speed limit tells you a maximum speed you must not exceed, set by the relevant authority.  You are legally not allowed to go faster than this.

It is not the same as either

(a) The speed you should be at which may be much less than the limit, depending on driving conditions

or

(b) The speed that vehicles are believed to travel at in practice

In the same way, the exposure limit is the maximum exposure you must not exceed, set by the relevant authority (not the manufacturer).  You are legally not allowed to have a higher exposure than this.

It is not the same as either

(a) The exposure you should be at which may be much less than the limit, depending on what is reasonably practicable

or

(b) The exposure that is believed to occur in practice

So the exposure limit does not represent how much exposure the manufacturer or anyone else thinks is going to occur.  

There may however be some information on likely exposure in normal use in the "extended" safety data sheet (should you manage to obtain this) which contains assessments of exposure scenarios.

DNEL = Derived No Effect Limit..yes it is sometimes published on SDS but can be calculated by Occupational Hygienist/ Toxicologist...in essence you can equate the DNEL to the OEL..

bit off topic and track of this thread...DNEL is required to be calculated for some but not all exposure patterns...Acute short term local effect cannot always be calculated, dermal in your example, so it may be approriate to calculate a Derived Minimum Effect Level (DMEL)...I could go into a log rant but it isn't really for here about how this does and doesn't work in all cases - we deal with development products worldwide that don't really have a lot of study information to base it on so sometimes it is a right wing view based on experience of similar drugs/exposure routes...we are talking what I would call step 2/3 assessments here step 1 being COSHH which is beyond the scope of this discussion